The N-methyl-D-aspartate (NMDA) class of excitatory amino acid receptors have rapidly become an important, major area of research in neuroscience. NMDA receptors not only mediate and modulate neurotransmission at a vast number of CNS synapses, but also play a pivotal role in linking neuronal activity to synaptic plasticity during development and learning. NMDA receptors have also been shown to be a critical factor in a variety of pathological processes and have been suggested to be causal in Huntington's and Alzheimer's diseases, schizophrenia, autism, cerebral palsy, and epilepsy development. It is now becoming increasingly apparent that there are multiple populations of NMDA receptors. At physiological, toxicological, and biochemical levels of analysis, there is evidence that there are at least two distinct populations of NMDA receptors that differ in their anatomy, pharmacology, molecular composition, and development. In addition, radioligand binding studies indicate that there are at least two anatomically and pharmacologically-distinct binding site populations of NMDA recognition sites. Thus, NMDA receptors appear to be homologous to the other, genetically-related (Myers et al., 1989) ion-channel receptors (nicotinic, GABA-A, and glycine) in having multiple, genetically-related forms (isoforms or isoreceptors) which display differing distributions in the brain, differing developmental patterns of expression, and variations in agonist and antagonist sensitivities. NMDA receptor subtypes may have significant clinical implications, because the receptor form with the greater agonist sensitivity would be expected to be primarily responsible for the cell death resulting from modest elevations of extracellular glutamate following ischemia and hypoglycemia. The focus of this proposal is to identify the distinguishing properties of NMDA receptor subtypes and to determine how the differing measures of heterogeneity are inter-related. Quantitative autoradiography will be used to evaluate how the anatomically-distinct NMDA binding site populations differ in their physio-chemical ligand binding properties, their pharmacological properties, and their anatomical and ontological patterns of expression. These data are necessary for evaluating the correspondence between heterogeneity observed in physiological/toxicological studies and in radioligand binding studies. Anatomical and pharmacological properties of NMDA receptor proteins labelled by the photoaffinity ligand azido[3H]- MK801 will permit correlating NMDA receptor heterogeneity to that seen at the molecular level. Together these studies should provide a unifying classification and description of NMDA receptor subtypes. The identification of distinct receptor subpopulations is necessary not only for the understanding of NMDA receptor action but is also of fundamental relevance to the general area of NMDA-receptor mediated seizure activity and neurotoxicity. Only with the resolution of subtypes and their distinguishing properties, it is possible to determine their relative contributions to various aspects of normal and abnormal brain function and to develop subtype-specific antagonists that maximize protection while not interfering with normal functions.